PHYSICAL FEATURES OF THE IRISH SEA. 15 



Sample bags of all the more important sub-marine deposits we have come upon have 

 been sent, at Sir Archibald Geikie's request, to the Museum of the Geological Survey 

 in Jermyn Street. They have been examined there by Mr. Clement Reid, F.G.S., who 

 writes as follows in regard to them : 



" On comparing these samples with British deposits of Tertiary date, one finds a 

 marked difference in lithological character. Dredgings from the Irish Sea, and also from 

 the North Sea, are characterised by a much coarser and more gravelly texture than one 

 would expect at such depths coarser, in fact, than one finds in Pliocene deposits yielding 

 a similar fauna, indicating similar or even smaller depths. A glance at these dredgings 

 shows the reason for this, for they are largely composed of unworn or little worn 

 fragments of rock, often entirely encrusted by organic growth. The stones evidently 

 have not been transported far by water, or they would be well rounded, like the pebbles 

 found in our Eocene beds. The incrusting organisms show also that the fragments have 

 lain undisturbed on the sea-bed, yet they have been derived from far-distant sources. 

 Though no Glacial striae were observed, and no undoubted sub-fossil Arctic shells have 

 yet been found at these localities, yet there seems little doubt that the bulk of the 

 material on the sea-bottom over this area has been derived from the breaking up of 

 pre-existing Glacial deposits. This may occur at a depth of several fathoms through 

 the gradual washing away of the muddy and sandy matrix of a boulder clay or Glacial 

 gravel. Coarse gravel is thus caused to accumulate at a spot where the currents may be 

 too feeble to transport anything but sand. 



" This sub-marine origin of angular gravel deposits should not be forgotten, for it 

 affects the lithological character of the sea-bottom over most of the area which was 

 formerly glaciated, even as far south as Cornwall. On the other hand, it does not affect, 

 except to a small extent, the sea-bed beyond the former limit of the ice, and it does not 

 affect pre-Glacial deposits. Thus we must always expect to find at similar depths the 

 same fauna associated with deposits of finer texture as soon as we leave the glaciated 

 area, or when we go back into Tertiary times. 



" It is also worth noting that the occurrence of a stony bottom at 20 or 30 

 fathoms where normally there would be no deposit coarser than sand will probably lead 

 to a disproportionate increase of all incrusting organisms, and of all organisms needing a 

 solid base. This has certainly taken place, as anyone studying our shoal-water Tertiary 

 deposits will have observed. They contain few stones, and though each stone or dead 

 shell may be covered with incrusting organisms, yet the relative proportion of these to 

 the free forms is far smaller than seems commonly to be the case in the seas that now 

 wash our shores. The sole exception to this rule among the British Tertiary strata is 

 found in the Coralline Crag, in which the contemporaneous consolidation of the limestone 

 was sufficient to provide the necessary solid base for the incrusting and fixed organisms 

 so abundant in that deposit. 



" Notwithstanding this peculiarity of most of the dredgings, a few samples may well 

 be compared with our Older Pliocene (Coralline Crag). I would particularly draw attention 

 to certain localities where material almost entirely of organic origin has been obtained. 

 Of these perhaps the most interesting are some samples full of Cellaria fstnlosa (found to 

 the south-east of the Calf Sound, 20 fathoms). They are in many respects strikingly like 

 certain parts of the Coralline Crag. The more ordinary type of Coralline Crag, with its 



